Seismic surveying



UNITED S'I'TES TENT OFFHC SEISMIC SURVEYING `lohn E. Owen, Tulsa, Okla., assignor to Geophysical Research Corporation, New York, N. Y., a corporation of New Jersey Original application May 27, 1938, Serial No. 210,349. Divided and this application October 18, 1938, Serial No. 235,577

9 Claims. (Cl. 181-05) 4D 2 A. S111 w t (A-) sin wf where The condition of balance obtained by making A maximum amplitude of the vertical component of the motion Am B maximum amplitude of the horizontal w Component gf the motion which condition is again obtained by adjusting hortzontail dlsplacement 3119 me t the relativeY amplitude of the two components. 2 vertlcal dlsplacement at me t The two geophones are acted upon by the rez 2M Where f 2 frequency of motion m flected waves coming nearly vertical from below, Cycles per second' but only the vertical geophone is responsive to This invention relates to seismic surveying.

In the reflection wave method of seismic surveying, the reflection wave record is complicated by the eiects produced by surface waves generated at the explosion of the shot unless provision is made for elimination of such eects from the record.

An object of this invention is an apparatus for eliminating the objectionable effects of surface waves from a reflection wave record.

The surface Waves above referred to are the waves commonly known as "ground roll and are of large amplitude and low frequency and travel out from the shot point along the surface of the earth. These waves are closely analogous to the surface waves known as Rayleighfwavcs which are a well-known type of wave produced as a result of natural earthquakes. The earth particles executing such a wave motion move over an elliptical path and the motion of a particle is retrograde with respect to the direction of propagation of the wave.

According to the present invention, two geophones are provided at the recording station, and are arranged close together so as to be substantially simultaneously actuated by the earth motion. One geophone is in a vertical position and the other in a horizontal position with its axis pointed toward the shot point. The first or vertical geophone will respond only to vertical motion while the second or horizontal geophone will respond only to horizontal motion. The displacement of both geophones when actuated by ground roll is the vector sum of a horizontal and a vertical component of displacement out of phase. Since the motion is elliptical, the two components of the displacement may be represented by IB=BCOSwt Since the vertical geophone responds only to the vertical component, its output Will be pro portional to A sin o t. Similarly, the output of the horizontal geophoneswill be proportional t@ B cos w t.4

The output from the horizontal geophone is fed through an electrical circuit which performs upon it an operation equivalent to differentiating, so that the altered output will be proportional to the time derivative of B cos w t which is '-B o sin w t. This altered output is then combined with the unaltered output of the vertical geophone. 'I'he sum of the altered output of the horizontal geophone and the output of the vertical geophone is expressed as follows:

The eect of the surface Waves is completely eliminated by making the combined output equal to zero which is accomplished by making A B w which in turn is very simply accomplished merely by adjusting the relative amplitudes of the two components by means of suitable controls on the recording amplifiers. I The above-described balancing may be accomplished equally well by running the output from one of the geophones through an electrical circuit which performs an operation equivalent to integrating and then combining such altered output with the output of the other geophone. For example, by integrating B cos w t which represents the output of the horizontal geophone, there is obtained the expression E sin wt w By combining the integrated output of the horizontal geophone with the output of the vertical geophone, but out of phase, such combined output is expressed as follows:

e=A sin it-g sin vmt such waves. The reiiected waves are therefore recorded in the usual fashion while the ground roll is balanced out.

1t is obvious from the foregoing description that a balance can be obtained for only a single i frequency by the arrangement just described and that this frequency may be adjusted tothe predominating frequency of the ground roll merely by varying the amplitudes of the two geophone outputs. Usually, most of the energy contained in ground roll does reside in a band close to one predominating frequency so that the above system works quite Well. It is true, however, that ground roll is transient in nature and for that reason does carry energy in a large number of frequencies so that to balance it out completely a more elaborate system must be used. In such more elaborate system, the output from one of the geophones is fed into a phase shifting network, which is so designed that it shifts the phase of practically all frequencies contained in a wave fed through-it by approximately 90. The altered output thus obtained is then combined with the unaltered output of the second geophone and the relative amplitudes are adjusted to give a balance.

Other objects, novel features and advantages of this invention will become apparent from the following specification and accompanying drawing, wherein:

Figs. 1, 2, 3 and 4 are diagrammatic illustrations of different embodiments of the apparatus of the invention.

In the embodiment illustrated in Fig. 1, two geophones II and I2 are provided at the receiving station of which II is arranged in a horizontal plane with its axis pointing toward the shot point and the geophone I2 is arranged vin a vertical position. The ltwo geophones are placed as close together as possible. The output of the geophone II is connected through a voltage divider I3 across the input of a vacuum tube` input circuit of a vacuum tube amplifier 23. Theoutput of the amplifier I3 contains a winding 24 and the output circuit of the vacuum tube amplifier 23 contains a winding 25, which two.

windings are arranged in series-aiding relation and constitute the primary of a transformer 26. 'I'he secondary winding of the transformer 26 is connected to a recording galvanometer 28.

'I'he system illustrated in Fig. 2 is similar to that illustrated in Fig. l, except that the differentiatng network I6 of Fig. l is replaced by the integrating network I6', consisting of a resistance I8 bridged by a condenser I1.

Considering the dierentiating network in Fig. 1, the input voltage e1 (t) produces a current through the condenser I1 and resistance I8 which may be termed i (t). The reactance of the condenser I1 is very large compared with the resistance I8 so that the current i (i) is determined primarily by the value of the condenser I1, i. e., it will be approximately 90l out of phase with the exciting voltage and will be proportional to frequency. 'Ihe output voltage e2 (t) being equal to i (t) R, (R being the value of the resistance I6), will, therefore, be also 90 out of phase with e1 (t) and will be proportional to frequency. In other words, the voltage e2 (i) will approximate the time derivative of the voltage e1 (t).

In the integrating network of Fig. 2, the values of I1 and I8 are so proportioned that for the frequency involved the value of the resistance I8 is high compared with the reactance of the condenser ,I'I. The current i (t) flowing through the circuit will then be very .nearly in phase with the exciting voltage and approximately equal in value to (R being the value of resistance I8). The voltage, e2 (t), across the condenser is then est CiRfdOd't, approximately l (C being the reactance of the condenser I1).

In the system illustrated in Fig. 3, the output of the horizontal geophone 3I is connected through a phase shifting network 33 to the input of the vacuum tube amplifier 34. This network is the same as that shown and described in an article byv O. J. Zobel in the Bell System Technical Journal, Volume VII, No. 3, page 514. By properly choosing the constants'of this network, it may be made to give a phase shift of approximately 90 for all frequencies labove an arbitrarily assigned frequency which, for the purpose of this invention, may be set somewhere in the neighborhood of cycles. The phase shift is obtained, with a practically at frequency characteristic or, in other words, there is no appreciable frequency distortion. The output of amplifier 34 is connected through a transformer 35 and voltage divider 36 across the input of a vacuum tube amplifier 31 having a winding 38 in its output. The vertical geophone 32 is coupled through the intermediary of the resistance 39 to the input of a vacuum tube amplifier 40. The output of the vacuum tube amplifler is connected through the transformer 4I and voltage divider 42 to the input of a vacuum tube amplifier 43. In the output of the vacuum tube amplifier 43 is provided a winding 44 which is in series-aiding relation to the Winding 38 with which it constitutes Ythe primary of a transformer 45, the secondary 46 ofA which is connected to a recording galvanometer 41.

The amplifiers I9 and 23 of Figs. 1 and 2 are properly adjusted to equalize the amplitude of the amplified and altered surface wave component of the geophone outputs so that these components balance each other out and have no effect upon the recorder. Ihe amplifiers 31 and 43 of Fig. 3 are likewise properly adjusted to have the same effect.

' In the system illustrated in Fig. 4, 5I is either a displacement geophone or a velocity geophone arranged in such position as to be responsive only to horizontal motion while 52 is either a. velocity geophonev or an acceleration geophone arranged to be responsive onlyto vertical movement. The output from the geophone 5I is impressed through the voltage divider 53 upon the input of an amplifier 54 which may consist of one or more stages of amplification. The output geophone 52 is impressed through the voltage divider 55 upon the input of an amplifier 56 which may consist of one or more stages of amplication. The outputs of the two amplifiers 54 and 56 are impressed through the transformer k 51 on the recorder 58.

In the above-described system, one geophone may act as a displacement meter for the range of frequencies contained in the ground roll While the other may act as a velocity meter for the same range of frequencies. Since velocity is the time derivative of displacement and conversely displacement is the time integral of velocity, the outputs of these geophones after suitable adjustment of amplitude may be added directly together to obtain elimination of ground roll in the manner described. This modification has the advantage of simplifying the electrical circuit required in the recording channel by permitting the omission of the integrating or diiferentiating circuit. Also, in the system just described, the velocity type geophone may be paired with a geophone the response of which is proportional to the acceleration of the earths motion. In this case, the same effect will be obtained, since acceleration is the time derivative of displacement and conversely velocity is the time integral of acceleration.

'I'he term geophone is meant to include all types of devices which translate earth vibrations into electrical waves. Although the horizontal and vertical geophones have been spoken of as being two separate instruments, in practice the horizontal and vertical elements are mounted in one case for convenience and the combined instruments may then be known as a two-component geophone. Furthermore, although the specification describes the operations of integration, diierentiation or phase shifting as being applied to the output of the horizontal geophone and since the performance of these operations will in no way affect the character of the reflecltion impulses which are picked up by the vertical geophone, nevertheless in some cases, these operations may be performed on the output of the vertical geophone instead of on the output of the horizontal geophone.

It is of course understood that various modifications may be made in the apparatus above described, without in any way departing from the spirit of the invention as defined in the appended claims. Furthermore, it is intended that the claims shall cover all such modifications as fall within the scope of their terminology. The recorder 25 illustrated in the drawing is of the type which makes but a single trace and in the claims the word recorder is used to mean such type device.

This application is -a division oi applicants co-pending application, Serial No. 210,349, Seismic surveying, led May 27, 1938.

I claim:

1. Apparatus for receiving and recording articial seismic waves comprising a pair of geophones arranged so as to be substantially simultaneously actuated by earth motion, one of said geophones being responsive only to the horizontal component of said earth motion and the other geophone being responsive only to the vertical component of said earth motion, a single recorder, connections between both said geophones and said recorder for impressing the outputs of both geophones on said recorder, and means in said connections for balancing out the effects of the surface waves received by said geophones.

2. Apparatus for receiving and recording artiflcial seismic waves comprising a pair of geophones arranged so as to be substantially simul- 3 taneously actuated by earth motion, one of said geophones being responsive only to the horizontal component of said earth motion and the other geophone being responsive ony to the vertical component of said earth motion, a single recorder, and connections between both said geophones and said recorder for impresssing the outputs of both geophones on said recorder, said connections including a network between one geophone and the recorder for altering the surface wave output of said geophone to make it proportional to the same trigonometric function as, but of opposite sign to, the surface wave output of the other geophone.

3. Apparatus for receiving and recording artificial seismic waves comprising a pair of geophones arranged so as to be substantially simultaneously actuatedby earth motion, one of said geophones being responsive only to the horizontal component of said earth motion and the other geophone being responsive only to the vertical component of said earth motion, a single recorder, and connections yfor impressing the output of each geophone on said recorder, said connections including an integrating network between one geophone and the recorder for altering the surface wave output of said geophone so that it is proportional to the same trigonometric function as, but of opposite sign to, the surface wave output of said other geophone.

4. Apparatus for receiving and recording artiflcial seismic waves comprising a pair of geophones arranged so as to be substantially simultaneously actuated by earth motion, one of said geophones being responsive only to the horizontal component of said earth motion and the other geophone being responsive only to the vertical component of said earth motion, a single recorder, and connections for impressing the output of each geophone on said recorder, said connections including a differentiating network between one geophone and the recorder for altering the surface wave output of said geophone so that it is proportional to the said trigonometric function as, but of opposite sign to, the surface wave output of said other geophone.

5. Apparatus for receiving and recording articial seismic waves comprising a pair of vgeophones arranged so as to be substantially simultaneously actuated by earth motion, one of said geophones being responsive only to the horizontal component of said earth motion and the other geophone being responsive only to the vertical component of said .earth motion, a single recorder, and connections for impressing the output of each geophone on said recorder, said connections including a phase shifting network between one geophone and the recorder for altering the surface wave output of said geophone so that it is of the same amplitude but 180 out of phase with the surface wave output of the other geophone.

6. Apparatus for receiving and recording artiilcial seismic waves comprising a pair of geophones arrangcd so as to be substantially simultaneously actuated by earth motion, the first of said geophones being responsive only to the horizontal component of said earth motion and the second geophone being responsivev only to the vertical component of said earth motion, means for altering the surface wave output of one of said geophones so that it is proportional to the same trigonometric function as, but of opposite sign to, the surface wave output of the other geophone, means for amplifying and combining the altered geophone outputs with the other geophone output and a single recorder connected t0 said means.

'7. Apparatus for receiving and recording articial seismic waves comprising a pair of geophones arranged so as to be substantially simultaneously actuated by earth motion, the first of said geophones being responsive only to the horizontal component of said earth motion and the second geophone being responsive only to the vertical component oi said earth motion, means for converting the surface wave output of said first geophone so that it is proportional to the same trigonometric function as, but of opposite sign to, the surface Wave output of said second geophone, means for amplifying said geophone outputs and a single recorder connected to said amplifying means.

8. Apparatus for receiving and recording artificial seismic Waves comprising a pair of geophones arranged so as to be substantially simultaneously actuated by earthmotion, the rst of said geophones being responsive only to the horizontal component of said earth motion and the second geophone being responsive only to the vertical component of said earth motion, an electrical circuit for amplifying the output of said second geophone, a second electrical circuit for converting the surface wave output of said rst geophone so that it is of the same amplitude but 180 out of phase with the amplified surface wave output of said second geophone, and a single recorder connected to both said circuits.

9. Apparatus for receiving and recording articial seismic waves comprising a pair of geophones arranged so as to be substantially simultaneously actuated by earth motion, the first of said geophones being responsive only to the horizontal component of said'earth motion and the second geophone being responsive only to thevertical component of' said earth motion, an electrical circuit for amplifying the outputof one geophone, a second electrical circuit for con-v verting the surface wave output of the other geophone so that said outputs are of the same amplitude but'180" out of phase and a single recorder connected to both circuits. y

JOHN E. OWEN. 

